This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The scientific community has long been troubled by the presence of nonspecific small molecules that inhibit proteins other than their hypothesized targets. The behavior of a large and diverse subset of these inhibitors can be explained by a common mechanism: aggregation. At micromolar concentrations some nonspecific molecules form large aggregates that inhibit a wide range of unrelated soluble proteins. From previous studies we know that inhibition results from the direct interaction of protein and aggregates. This interaction is reversible and does not result in denaturation, but the extent of our understanding stops there. How does binding to the aggregate lead to inhibition? Does the aggregate bind to a particular protein surface or is it nonspecific? Does binding restrain or induce dynamic motion in the protein? I will use deuterium exchange mass spectrometry to address these questions. Mass spectrometry coupled to deuterium exchange of amide hydrogens provides evidence of how exposed certain regions are to solvent. This allows us to examine the accessibility of surface protons and the overall dynamics of the enzyme through the exchange of core (or buried) protons. Additionally, we will be able to determine whether there is a particular surface to which the aggregate binds. There are few techniques that allow this level of sensitivity and among them mass spectrometry is the most practical. These studies will provide insight into the mechanism of promiscuous inhibition by aggregation, thus elucidating a novel interaction between small molecules and their biological targets.